Boiler plants, particularly those which generate heat through the combustion of coal, are well known sources of air and water pollution. The emissions of sulfur from these plants have been well-documented as a major contributor to the problem of "acid rain". Various toxic or otherwise undesirable compounds are also produced during the combustion of coal, these including uranium, beryllium, chromium, barium, arsenic, selenium, mercury and coal tar.
The worsening environmental impact of the above described pollutants has become increasingly apparent in the last decade. Thus, considerable resources have been expended to come up with reliable and environmentally acceptable ways of removing the pollutants from flue gas emissions.
The currently available techniques for removing these pollutants include cooling the flue gas down to condensation within a heat exchanger that heats boiler feedwater. The prior art, for example, recognizes that cooling flue gases that contain sulfur trioxide and water vapor will result in condensation of sulfuric acid. See U.S. Pat. No. 4,526,112 to Warner; U.S. Pat. No. 4,874,585 to Johnson et al; and U.S. Pat. No. 4,910,011 to Dorr et al.
Although the aforementioned systems are more or less effective in removing some degree of the pollutants from the flue gas, they merely shift the pollution category from atmospheric waste to solid waste. Attempts have therefore been made to convert pollutants into useful products as they are removed from the flue gases.
One such approach involves the introduction of ammonia to combine with sulfur oxides in flue gases and form ammonium bisulfite and/or ammonium bisulfate.
This approach fails to address the presence of other pollutants in the flue gas, such as nitrogen compounds, uranium, beryllium, chromium, barium, arsenic, selenium, mercury and coal tar. Further, optimum transfer of heat to the boiler feedwater requires the utilization of densely packed tubes through the heat exchanger. Thus, the approach as described above is inefficient and commercially impracticable because the dry products produced thereby have a tendency to buildup on the tubes and to severely impede the flow of flue gas.
It has been further recognized in the prior art that SO.sub.2 is not readily oxidized to SO.sub.3 despite the presence of sufficient atmospheric oxygen in the flue gas to react therewith. In response to this recognition, the prior art has introduced oxidizing agents such as hydrogen peroxide (H.sub.2 O.sub.2) dissolved in water in order to precipitate H.sub.2 SO.sub.4 from the sulfur dioxide within the heat exchanger. See, for example, U.S. Pat. No. 4,783,326 to Srednicki. However, the added cost of introducing the required amount of oxidizing agents make such systems economically unattractive as well as complex.
It is therefore an object of the present invention to provide a system for economically removing pollutants from flue gas by which substantially all oxides of sulfur and nitrogen, as well as other harmful contaminants are removed.
It is a further object of the invention to provide such a system in which the oxides of sulfur and nitrogen are not merely removed from the flue gas to be disposed elsewhere, but are converted into useful and environmentally safe substances, particularly ammonium sulfate, ammonium bisulfate, and ammonium sulfite.
It is another object of the present invention to provide such a system in which the formation of the useful byproducts does not hamper the proper functioning of the apparatus.